The advance of multimedia in recent years has enabled even image and sound data to be processed with digital signals. In general, images and sounds have a large amount of information, and, therefore, a high capacity/high density recording medium is required to cope with this. Optical disks, which can also cope with a demand for a further increase in capacity and a further increase in density in the future, such as DVD (digital versatile disk) and CD (compact disk), have attracted attention as the above recording medium.
The optical disk, in many cases, comprises two transparent substrates bonded to each other through an information recording layer. Methods for bonding the two substrates are disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 1-266447, 61-292244, and 63-49424 and "Denshi Zairyo (Electronic Materials)," Vol. 35, No. 6, 46-49, 1996), "Pressure Sensitive Adhesive Double Coated Tape used in Bonding of DVD" In particular, in the method described in the journal "Denshi Zairyo," a pressure sensitive adhesive double coated tape is applied to one of or both the two substrates, and the two substrates are then intimately contacted with each other, followed by press bonding to prepare an optical disk.
As is reported by "Denshi Zairyo", for the above method using a pressure sensitive adhesive double coated tape, various improvements are necessary from the viewpoint of workability. Specifically, in this method, separation of a release liner and cutting of the pressure sensitive adhesive double coated sheet into a desired shape are necessary in addition to the application of the pressure sensitive adhesive double coated tape, complicating the process, which tends to increase the production cost. Further, this method has a further drawback that, in the application of the pressure sensitive adhesive double coated sheet, the presence of a residual stress creates warpage of the substrate.
On the other hand, typically, Japanese Unexamined Patent Publication (Kokai) No. 8-161771 discloses a method not using a pressure sensitive adhesive double coated sheet. Specifically, in this method, instead of using the pressure sensitive adhesive double coated sheet, two UV-permeable substrates are bonded to each other through an ultraviolet (UV) curable adhesive precursor formed by roll coating, and UV is applied to the adhesive precursor through the substrate to form a transparent adhesive layer.
Among various conventional optical disks, recently commercialized one are dual layer type optical disks which are also referred to as "DVD-9". In this type of optical disk, two information recording layers, one of which is of a reflection type and the other a semitransparent type, are provided on substrates through a transparent adhesive layer, permitting recording of information on the information recording layer and reading of the information recorded on the information recording layer to be performed from one side of the disk. When this dual layer type optical disk is produced by the method described in Japanese Unexamined Patent Publication (Kokai) No. 8-161771 noted above, the adhesive layers would be formed by applying UV to the pressure sensitive adhesive precursor through the semitransparent recording layer. Generally, the adhesive layer in such optical disk is used to attain the bonding of the substrates, and therefore the adhesive layer should be constructed, for example, so that a laser beam from an optical head is passed through the semitransparent recording layer and focused on the underlying reflection type recording layer. That is, what is required of the adhesive layer is not only to be homogeneous but also to function also as an optical spacer having a given thickness.
However, as is pointed out in Japanese Unexamined Patent Publication (Kokai) No. 8-306077, the adhesive precursor of the optical disk disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-161771 has a problem that unevenness of thickness of the adhesive layer is produced in the radial direction of the optical disk. To apply UV to the adhesive precursor through the semi-transparent recording layer, an output of UV light source has to be increased to a higher level than would be needed, in view of absorption of UV in the recording layer. Therefore, there is a possibility that the optical disk obtained has deteriorated optical properties and yield.
In addition to the above problems, as is pointed out in Japanese Unexamined Patent Publication (Kokai) No. 9-320129, the adhesive precursor of the above-described type has a problem that bubbles can be incorporated into an adhesive layer due to presence of air in an atmosphere, when an adhesive precursor is UV-cured to form the adhesive layer for bonding the substrates.
Incorporation of bubbles in the adhesive layer may adversely affect the qualities of the resulting optical disks and therefore, as is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 61-292244, it is conceived to bond the substrates through an adhesive layer in vacuum or under the environmental conditions of reduced pressure. In practice, the bonding of the substrates in accordance with the above method is effective to prevent air from being captured between the substrates, however, it cannot fully solve the above-mentioned problem concerning the incorporation of the bubbles. This is because the incorporation of the bubbles in the adhesive layer can be caused due to the adhesive used itself, in addition to the above-described reason, i.e., atmospheric air. The inventors have found that the adhesive which is generally used as a liquid adhesive contains a substance capable of being evaporated or vaporized in the process of curing of the adhesive, and therefore finely divided bubbles can be formed in the adhesive layer as a result of the evaporation or vaporization of said substance in the vacuum or under the reduced pressure, and also the thus formed bubbles cannot be again dissolved and dissipated in the adhesive layer.
Particularly important problem is caused when such incorporation of bubbles is observed in the single side dual layer type optical disks in which a single side or surface is used for the reproduction purpose. For the dual layer type optical disks, two disk substrates are generally bonded through a transparent adhesive layer, and the adhesive layer is also used to space the substrates for optically separating two recording layers of said substrates, and therefore incorporation of fine and visually uninspectable bubbles in the adhesive layer must be fully prevented, however, such prevention of the bubble incorporation problem causes another problem concerning production technique. Accordingly, it is desired to completely remove the bubble formation problem due to the above-described different reasons from the adhesive layer which is used to bond two disk substrates without, causing another problem.